Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust system. Although these three component control catalysts work quite well after they have reached operating temperature of about 300.degree. C., at lower temperatures they are not able to convert substantial amounts of the pollutants. What this means is that when an engine and in particular an automobile engine is started up, the three component control catalyst is not able to convert the hydrocarbons and other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. However, California has recently set new hydrocarbon standards (these standards most probably will be promulgated nationwide) which can not be met with the current state of the art three component control catalysts.
Applicants have found a solution to this problem which involves the use of an adsorbent bed to adsorb the hydrocarbons during the cold start portion of the engine. Although the process will be exemplified using hydrocarbons, the instant invention can also be used to treat exhaust streams from alcohol fueled engines as will be shown in detail. Applicants' invention involves taking the exhaust stream which is discharged from an engine during the initial startup of the engine (cold start) and diverting it through an adsorbent bed which preferentially adsorbs hydrocarbons over water under the conditions present in the exhaust stream. The exhaust stream discharged from the adsorbent bed (first exhaust stream) is flowed over a primary catalyst and then discharged into the atmosphere. After a certain amount of time, the adsorbent bed has reached a certain temperature (about 150.degree. C.) at which the bed is no longer able to remove hydrocarbons from the engine exhaust stream. That is, hydrocarbons are actually desorbed from the adsorbent bed instead of being adsorbed. At that point the engine exhaust stream is diverted such that the engine exhaust stream completely bypasses the adsorbent bed and flows directly over the primary catalyst bed.
After an additional amount of time during which the primary catalyst has reached its operating temperature, the engine exhaust stream is divided into a major and minor portion. The major portion of the engine exhaust stream is flowed directly over the primary catalyst while the minor portion of the engine exhaust stream is flowed over the adsorbent bed thereby desorbing the hydrocarbons and any other pollutants that were adsorbed on the bed. The stream from the adsorbent bed (second exhaust stream) is again flowed over the primary catalyst and then discharged into the atmosphere. When all the hydrocarbons have been desorbed from the adsorbent bed, the engine exhaust stream is completely directed over the primary catalyst. This ensures that the adsorbent bed is not exposed to high temperatures which may damage the adsorbent bed.
The adsorbents which may be used to adsorb the hydrocarbons may be selected from the group consisting of molecular sieves which have 1) a Si:Al ratio of at least 2.4;2) are hydrothermally stable; and 3) have a hydrocarbon selectivity greater than 1. Examples of molecular sieves which meet these criteria are silicalite, faujasites, clinoptilolites, mordenites and chabazite. The adsorbent bed may be in any configuration with a preferred configuration being a honeycomb monolithic carrier having deposited thereon the desired molecular sieve.
The prior art reveals several references dealing with the use of adsorbent beds to minimize hydrocarbon emissions during a cold start engine operation. One such reference is U.S. Pat. No. 3,699,683 in which an adsorbent bed is placed after both a reducing catalyst and an oxidizing catalyst. The patentees disclose that when the exhaust gas stream is below 200.degree. C. the gas stream is flowed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200.degree. C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion, the major portion being discharged directly into the atmosphere and the minor portion passing through the adsorbent bed whereby unburned hydrocarbons are desorbed and then flowing the resulting minor portion of this exhaust stream containing the desorbed unburned hydrocarbons into the engine where they are burned.
Another reference is U.S. Pat. No. 2,942,932 which teaches a process for oxidizing carbon monoxide and hydrocarbons which are contained in exhaust gas streams. The process disclosed in this patent consists of flowing an exhaust stream which is below 800.degree. F. into an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and then passing the resultant stream from this adsorption zone into an oxidation zone. When the temperature of the exhaust gas stream reaches about 800.degree. F. the exhaust stream is no longer passed through the adsorption zone but is passed directly to the oxidation zone with the addition of excess air.
Finally, Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol fueled automotive vehicle. This method consists of directing the cool engine startup exhaust gas through a bed of zeolite particles and then over an oxidation catalyst and then the gas is discharged to the atmosphere. As the exhaust gas stream warms up it is continuously passed over the adsorption bed and then over the oxidation bed.
Applicant's invention differs in several ways from the processes described in the prior art. First, the adsorbent bed used in applicant's process is a selective adsorbent bed which is a molecular sieve bed. What this means is that hydrocarbons and other pollutants are preferentially adsorbed over water which means that the adsorbent bed does not have to be very large in order to adsorb sufficient quantities of hydrocarbons and other pollutants during engine startup. Another distinguishing feature is that when the adsorbent bed exceeds a temperature of about 150.degree. C., the engine exhaust stream is diverted completely away from the adsorbent bed and routed directly over the primary catalyst. Once the three component control catalyst bed reaches the desired operating temperature, the exhaust stream is divided into a major and minor portion with the minor portion being flowed over the adsorbent bed, thereby desorbing the hydrocarbon and any other pollutants adsorbed thereon, while the major portion is directly flowed over the catalyst. Additionally, no excess air or oxygen is added to the catalyst. Applicant's process has the advantage of allowing the three component control catalyst to warm up much faster because the size of the adsorbent bed is minimized. That is, because the molecular sieves used in the adsorbent bed selectively adsorb pollutants over water, the volume of the adsorbent bed is much smaller versus adsorbents in the prior art which do not selectively adsorb pollutants. A smaller adsorbent bed means a smaller heat sink which means that a hotter exhaust gas stream contacts the catalyst. The molecular sieves which are used as the adsorbents also exhibit good hydrothermal stability, thereby minimizing replacement of the adsorbent bed.